1. Field of the Invention
The present invention relates to method of manufacturing refractive index distribution type lenses having large numerical aperture and a low aberration.
2. Description of the Prior Art
In optical fiber communication systems, it is desirable to optimize coupling between a light source, such as a semiconductor laser or a light emitting diode, and an optical fiber serving as a transmitting path. Generally, light from the source is focused by using an optical element such as a lens, or the like, to thereby improve coupling efficiency. As the optical element, spherical lenses, rod lenses of the gradient refractive index type, planar microlenses and the like have been proposed.
To manufacture various gradient refractive index planar lenses, for example, ions for causing a different refractive index are diffused into the surface of glass through ion exchange. That is, a lens portion having a relatively large refractive index is obtained by using this method and therefore it is possible to manufacture a gradient refractive index type lens having a large numerical aperture.
A lens 1 having a refractive index gradient in the plate is provided with a semi-cylindrical refractive index gradient region 2 as shown in FIG. 1. In region 2, the refractive index ideally varies according to a quadratic curve from in the plane surfaces in the X- and Z-directions, and is constant in the Y-direction, as shown in FIG. 2. However, it is not easy to obtain an ideal refractive index distribution as expressed by the following formula: EQU n(x).sup.2 =n.sub.o.sup.2 (1-g.sup.2 x.sup.2) (1)
where n(x) represents a refractive index in the direction of the thickness, n.sub.o a refractive index on the slab surface, and g a quadratic constant of the distribution. That is, a lens manufactured by this method has a large aberration.